1. Field of the Invention
The present invention relates to an infrared photodetector and a method of manufacturing the same and, more particularly, an infrared photodetector which has a multiquantum-well structure forming a focal plane array and a method of manufacturing the same.
2. Description of the Prior Art
Quantum-well infrared photodetectors (QWIP) based focal plane arrays (FPA) are used for infrared imaging in 8 to 12 xcexcm wavelength range. Such multiquantum well (MQW) infrared photodetector is described in B. F. Levine, J. Appl. Phys. 74(8), Oct. 15, 1993, for example.
The quantum-well infrared photodetectors consist of a gallium arsenide (GaAs) substrate with epitaxially grown MQW structure of alternate layers of AlGaAs and GaAs. The MQW layers are lithographically patterned into mesas to form separate elements or pixels. An example of infrared photodetector having such MQW layers, is shown in FIGS. 1A and 1B.
A structure having an MQW layer 2 is formed on a GaAs substrate 1, the MQW layer 2 is partitioned by recesses 3 formed by the lithography, and semiconductor circuits (not shown) formed in a silicon substrate 9 are connected to partitioned portions. Thus, each area partitioned by the recesses 3 is a pixel PX.
Infrared radiation is incident from the back of the GaAs substrate 1. Incident radiation only has components Ey and Ex that are parallel with the surface of the GaAs substrate 1. A sectional structure of one pixel area of the infrared photodetector is shown in FIG. 2, for example.
In FIG. 2, an n-type layer 1a formed of GaAs, an MQW layer 2, and an n-type layer 3 formed of GaAs are formed sequentially on an n-type GaAs substrate 1. A diffraction grating 4 is then formed on an upper surface of the n-type layer 3. And, the diffraction grating 4 contacts a silicon substrate 9 via a bump 5 formed of indium. In this case, in FIG. 2, reference 6 denotes a recess which partitions neighboring pixels.
Radiation entering each pixel through the GaAs substrate 1 is detected by photo-induced electron transition caused in the MQW layer 2. The light received by one pixel of the infrared photodetector is converted into an electric current and then output to an electronic circuit (not shown) formed on the silicon substrate 9.
However, the MQW layer 2 absorbs only the radiation that has an electric field component perpendicular to the epitaxial plane. This means that for normally incident radiation to be absorbed, its direction needs to be changed within a detector""s pixel.
For changing direction of incident radiation, a diffraction grating etched on the top of the pixels as schematically shown in FIG. 2 is commonly used. The radiation diffracted from the grating 4 is confined to the pixel by total internal reflection.
Different schemes to facilitate absorption of the light, i.e., optical coupling, in QWIP were reviewed by the above mentioned article by Levine.
Progress began from simple method of MQW illumination through a substrate edge polished at 45xc2x0 angle used in the early work.
Coupling efficiency was initially improved by planar metallic strip gratings, which could diffract light at angles close to 90xc2x0.
Later, etched one-dimensional and two-dimensional periodic gratings with an internally reflecting layer on substrate further improved the efficiency to about four times that of 45xc2x0 coupling geometry, respectively.
As the two-dimensional periodic grating, as shown in FIG. 3A, there is a periodic grating 4. As shown in FIGS. 3B and 3C, one concave portion 4b is formed in the middle of one unit 4a by etching. A plurality of units 4a is formed in a matrix fashion, as shown in FIG. 3A.
In J. Appl. Phys. 71(7), Apr. 1, 1992, pp.3600-3610, Anderson et al. carried out detailed theoretical analysis and experimentally achieved, for 8-10 xcexcm long wavelength range, an efficiency of about 2-3 times that of 45xc2x0 coupling geometry. However, as shown in FIG. 3C, the periodic grating with internally reflecting layer 3 could produce only two passes through the MQW layer 2.
In Appl. Phys. Lett. 64(1994), pp.960-962, Sarusi et al. showed that increasing the number of passes through the MQW layer by employing a pseudo-random grating can achieve efficiencies about 14 times that of 45xc2x0 coupling. Experimentally, efficiency about eight times that of 45xc2x0 coupling was demonstrated for peak response wavelength 16.4 xcexcm. In their experiment light confinement to a pixel was achieved by thinning GaAs substrate.
The pseudo-random grating 4 has the structure which contains three stepped surfaces 4c , 4d , 4e each having a different height, as shown in FIGS. 4A to 4C, for example, and is formed by the two etched steps patterned. Optimized size for its unit cell was about 5.7 xcexcm and a smallest feature width was 1.25 xcexcm.
Later, Gunpala et al., in IEEE Trans. Electron Devices, 44 (1997), pp.45-50, used a pseudo-random grating for a QWIP device with 15 xcexcm wavelength. Also, in the article in IEEE Trans. Electron Devices, 44(1997), pp.51-57, Gunpala et al. used a pseudo-random grating for a QWIP device with 8.5 xcexcm wavelength. However, they could achieve only very low responsivity, e.g., at 8.5 xcexcm wavelength, the responsivity was 0.3 A/W and at 14 xcexcm wavelength, the responsivity was 0.4 A/W. This responsivity is slightly lower than twice that due to 45xc2x0 coupling. At the 8.5 xcexcm wavelength, optimized pseudo-random grating has unit cell of width of 2.9 xcexcm and smallest feature width of 0.4 xcexcm which is difficult to fabricate accurately.
Therefore, it has been thought that at 8.5 xcexcm wavelength, it is essential to use a periodic grating that has a smallest feature size larger than the pseudo-random grating mentioned above. These result in lowing the diffraction efficiency.
As mentioned above, the optical coupling scheme that has demonstrated highest optical coupling efficiency in QWIP uses a pseudo-random grating. However, the coupling scheme is difficult to implement at shorter wavelengths. Coupling scheme using crossed periodic gratings which normally result in lower coupling efficiency than that with pseudo-random grating is thought to be suitable for peak wavelengths around 8.5 xcexcm. An earlier scheme used angled surface on the pixel top combined with the reflection grating, the planar metallic gratings, the saw-tooth gratings, etc.
At any rate, if two-dimensional periodic diffraction grating shown in FIGS. 3A to 3C is employed, the high optical coupling capability as achieved in the pseudo-random gratings shown in FIG. 2 and FIGS. 4A to 4C cannot be attained since the light is reflected by the periodic grating by an angle of 90xc2x0 relative to the substrate surface (not shown) at the time of second diffraction.
However, if the pseudo-random grating employed for the 8.5 xcexcm wavelength is constructed by using the structure shown in FIGS. 4A to 4C, the number of steps is increased since two-step lithography is required, as described above. In addition, because the width of the lowest step area is small like about 0.4 xcexcm, it is difficult to pattern the width with high precision and it is difficult to align the patterns in the two-step lithography.
If the patterning precision is degraded, the optical coupling in the multiquantum-well infrared photodetector is lowered.
It is an object of the present invention to provide an infrared photodetector having a new diffraction pattern and a method of manufacturing the same.
The above subjects can be covered by providing an infrared photodetector comprising of a photoabsorption layer formed on a substrate having a multiquantum-well structure; and a diffraction pattern formed on the photoabsorption layer to have recesses, planar shape of each of which contains curved shapes and sectional shape of each of which has a single step shape.
Also, the above subjects can be overcome by providing a method of manufacturing an infrared photodetector comprising the steps of forming a photoabsorption layer having a quantum-well structure on a substrate; forming a light transmitting layer on the photoabsorption layer; forming a mask, on which patterns having curved shapes are formed, on the light transmitting layer; forming a diffraction pattern on the light transmitting layer by etching the light transmitting layer in areas which are not covered with the mask; and removing the mask.
According to the present invention, the diffraction pattern whose planar shape includes curves (e.g., elliptic curves) is formed on the photoabsorption layer of the multiquantum-well structure.
It has been checked that the optical coupling rate of the infrared rays of the wavelength of 8.5 xcexcm can be enhanced by such diffraction pattern. Besides, because the recesses constituting the diffraction pattern have a sectional shape like a single step, such recesses can be formed by a single lithography step and thus the fabrication steps can be reduced. Moreover, the minimum width of the diffraction pattern is about 0.6 xcexcm and thus pattern fabrication is easier.